Known metallic alloy slurry molding machines and associated assemblies may be used to mold a metallic alloy slurry such as, for example (but limited to) a slurry of magnesium, aluminum, and zinc, and any combination thereof, or equivalent thereof. The industry in general may refer to the metallic alloy slurry molding machine as a thixo-molding machine.
A first type of metallic material may exist in any one of two possible states: a liquefied state or a solidified state. The temperature at which the first type of metal material may change between the liquefied state and the solid state may be called the “melt” temperature. As a rule of thumb, the first type of metallic material may be a pure metal having substantially no impurities therein. For example, a cast molding or a die molding process and machinery may be used to mold the first type of metallic material by placing the first type of metallic material existing in the liquefied state into a mold assembly, cooling the mold assembly, and then removing the solidified first type of metallic material from the mold assembly,
In sharp contrast to the first type of metallic material, a second type of metallic material may exist in one of three possible states: the liquefied state, the solidified state and a slurry state. The temperature at which the second type of metallic material changes between the liquefied state and the slurry state may be called the liquefied-slurry change temperature. The temperature at which the second type of metallic material changes between the slurry state and the solidified state may be called the slurry-solid change temperature. The slurry-solid change temperature is less than the liquefied-slurry change temperature. The slurry temperature range is temperature between the slurry-solid change temperature and the liquefied-slurry change temperature. The second type of metallic material existing in the slurry state is a combination of the second type of metallic material in the liquefied state and the second type of metallic material in the solidified state. An approximate visual analogy of the second type of metallic material may be a cup of hot water containing peas therein.
As a rule of thumb, the second type of metallic material is a metallic alloy that contains two or more metallic elements and/or nonmetallic elements usually fused together or dissolved into each other. For example, a thixo-molding process and machinery may be used to mold the second type of metallic material by placing the second type of metallic material existing in the slurry state into a mold assembly, cooling the mold assembly, and then removing the solidified second type of metallic material from the mold assembly. The advantage of using the second type of metallic material in the slurry state is that the strength of the molded article is inversely proportional to the temperature of the slurry, in that the cooler the slurry temperature, the stronger the resulting molded article will be. The reasons for the inversely proportional strength phenomena are known. Also, shrinkage of the molded article is less likely to occur when using MAS having a lower temperature in the slurry temperature range, in which the reduced shrinkage factor may improve part integrity and strength.
Henceforth, the second type of metallic material existing in the slurry state within the slurry temperature range will be referred to as “a metallic alloy slurry”. The metallic alloy slurry exists in the slurry state includes a liquid component and a solid component. The industry also may refer to the metallic alloy slurry as “a thixotropic metallic material”, and the molding machine that handles the thixotropic metallic material is called a thixo-molding machine.
The thixo-molding machine may outwardly appear to resemble a plastic resin injection molding machine. However, there are many internal differences between these two types of molding machines. The thixo-molding machine receives, at room temperature, a collection of chipped metallic alloy (such as an alloy of magnesium) into a hopper mounted on top of the thixo-molding machine. The chips, which exist in a solid state, are then volumetrically loaded into a smaller hopper that is mounted directly to a barrel. A rotating screw mounted in the barrel is then used to meter the chips along a length of the barrel. The screw rotation produces a shearing action which means that the screw mixes and/or tears the chips. The barrel includes heaters which apply heat to the chips as they are mixed and/or sheared by the screw. The chips are then transformed from the solidified state into the metallic alloy slurry (MAS). The MAS is then forced past a shut-off valve and then injected into a cavity defined by a mold assembly. Once the MAS becomes solidified in the mold assembly, the solidified MAS is removed and trimmed. Generally, several advantages are realized when thixo molding is used, such as: greater process control, increased part-to-part consistency; lower porosity; ability to mold complex features; better surface finish; net shape parts; thin wall molding; and reducing/eliminating a the need for secondary operations.
Sometimes the shut-off valve may be called a nozzle or a dispenser. Generally, the shut-off valve defines a supply passageway therein for conveying the MAS. The shut-off valve has a tip which defines an opening. The opening communicates the MAS into the cavity defined by the mold assembly. Controlling the flow (that is: either preventing the flow when not desired and permitting the flow when desired) of the MAS is achieved by locally cooling the MAS that is located near or at the opening of the valve so that the localized MAS may be transformed from the slurry state into the solidified state. The localized solidified MAS forms what is commonly known as a “thixo plug”. During a shot build up cycle and with the thixo plug in place in the valve opening, the thixo-molding machine builds up a shot of MAS (that is MAS in the slurry state) behind the solidified thixo plug. The built-up shot of MAS remains under a shot build-up pressure. During an injection cycle, the thixo-molding machine increases the internal pressure of the MAS higher than the shot build-up pressure. The higher built-up pressure (the pressure within the barrel and the valve) is known as a “plug blow out” pressure. The plug blow out pressure is high enough to blow the thixo plug out from the valve opening and into the mold cavity, followed by free flow the MAS (existing in the slurry state) from the passageway of the valve. Once the mold cavity is filled, the thixo plug may be reformed in the valve opening by a cooling effect induced by a cooling structure located nearby the valve opening.
However, the thixo plug may impose an operator safety hazard if the mold assembly is not in position to receive a blown out thixo plug from the dispenser. The MAS (in slurry state) may splatter and splash over unsuspecting operators of the thixo-molding machine. Avoiding this hazard requires a very consistent thixo plug (in solid state) or a very good control and management of local thermal condition in the area where the thixo plug is formed so that any excess pressure in the melt channel will not accidentally expel or blow out the thixo plug when the mold assembly is opened. Should the thixo plug suddenly become molten when the mold is open (as a result of intermittently operating localized cooling effects), the MAS in slurry state may be uncontrollably discharged from the dispenser and onto operators of the thixo-molding machine.
U.S. Pat. Nos. 5,785,915, 6,355,197, 5,975,127, 6,027,328, 3,401,426 and 4,386,903 all disclose molten plastic resin dispensers used with a resin plastic molding machine; however, these patents do not teach, suggest or motivate the industry to use molten plastic resin dispensers for dispensing the MAS. The reason for this may be that there are material attribute or material characteristic differences between the MAS and the plastic resin, and those differences may hamper or discourage the deployment of plastic resin dispensers in a thixo-molding machine. For example, such differences between the MAS and the plastic resin are (but not limited to):                the melting point of the MAS may range from 400° C. to 700° C. which is substantially higher than that of plastic resin;        the thermal conductivity of MAS is much higher than that of plastic resin;        the compressibility of MAS is significantly less than that of plastic resin;        the corrosiveness and/or abrasiveness of MAS (while solidified as a thixo plug for example) is much higher than that of molten plastic resin;        high fluidity and low viscosity of MAS (relative to the molten plastic resin) cause the MAS to travel through much smaller gaps that may exist between structural components of the thixo-molding machine; and        spontaneous explosive reactivity of some types of MAS; for example, exposing magnesium to air will cause magnesium to burn explosively. In sharp contrast, plastic resin does not spontaneously combust when exposed to air.        
As can be appreciated from the foregoing list of material differences, while known plastic-resin compatible molding-machine valves work satisfactorily with plastic resin, they raise technical concerns when these types of valve are proposed for use with the thixo-molding machine. These raised concerns have presently shaped conventional wisdom which calls for avoiding the combination of known plastic resin dispensers with thixo-molding machines because the MAS imposes technological difficulties and uncertainties that may adversely affect the resin plastic dispenser used in a thixo-molding machine.
By way of example that shows the conventional wisdom pertaining to current thixo molding technology, U.S. Pat. No. 6,533,021 ('021) discloses a MAS dispenser, in which a mold for a metal hot-runner injection molding machine includes a movable mold plate, a fixed mold plate having a nozzle for injecting molten metal into said cavity, and a heating device disposed outside the nozzle for heating metal. A gate cut portion is situated in the nozzle between the heating device and the tip. A temperature measurement device is arranged adjacent to the gate cut portion for measuring the temperature of the metal in the gate cut portion. A heating control device is connected to the heating device for controlling a temperature of the nozzle based on the temperature measurement device. A heat insulation device is arranged on the nozzle to cover at least an area where the gate cut portion is formed. The '021 patent discloses a nozzle that operates by forming and melting a thixo plug. FIG. 8 shows the nozzle operating with a pin 41 in which the pin 41 forces a thixo plug back into a melt channel 11 where the thixo plug is re-melted to become part of the melt. It is interesting to note that the thixo plug is formed and used only once as a plug mechanism, and then for the next injection cycle, an entirely new thixo plug is formed and used. In other methods, the thixo plug is expelled from the channel by melt pressure and trapped in a thixo plug catcher. These methods may have problems. If the thixo plug reenters the melt channel, it may not fully melt before injection and thereby inconsistencies in the molded product may be experienced. Discharging the thixo plug from the channel can be a safety hazard if the thixo plug is inadvertently discharged when the mold is open. Also, the pressure required to discharge the thixo plug may vary from shot to shot and timing of the opening of the melt channel is difficult to predict. This can be a serious concern when making multiple drops into the mold assembly.
U.S. Pat. No. 6,357,511 discloses a thixo feed body (called a sprue bushing) which does not appear to teach a thixo dispenser, and appears to teach overcoming leaky spure connections.
Therefore, a solution is desired which addresses, at least in part, the above-mentioned and other potential shortcomings.